This invention relates to an intake air heating system for an internal combustion engine, and more specifically, to an improvement in an air heating passage which is defined around the outer periphery of an exhaust passage as a source of heated air for an intake air heating system having an air cleaner of the temperature regulating type.
Generally, in an internal combustion engine for use with automobiles, it has been well known to use an air cleaner of the temperature regulating type having a temperature regulating valve which functions to draw air heated in the vicinity of an exhaust passage of the engine into the engine when the temperature of outside air is low, whereby an optimal ratio of air-fuel mixture is provided by regulating the temperature of intake air in a case where the temperature of the outside air varies extremely, for example between summer and winter seasons.
One example of such air cleaners is illustrated in FIG. 1 in which an air heating passage 5 is defined around the outer periphery of an exhaust passage or an exhaust manifold 4 of an internal combustion engine B by means of a heat insulating plate or a manifold cover 3 and is connected through a heated-air induction pipe or a hot air duct 6 to an outside air induction duct 2 for an air cleaner 1. A temperature regulating valve 7 is provided at the junction between the valve 7 and the duct 2.
The temperature regulating valve 7 is opened and closed by a pressure actuator 8 which is communicated, through a temperature sensitive valve 9 attached to the air cleaner 1 downstream of air filter elements (not shown) located therein, with an intake passage 11 downstream of a throttle valve 10 so that negative pressure in the intake passage 11 is applied to the pressure actuator 8 in response to the temperature of intake air drawn into the air cleaner 1.
Specifically, during cold running of the engine at low outside air temperatures, the temperature sensitive valve 9 is opened to apply the decreased pressure in the intake passage 11 of the engine B to the pressure actuator 8 so that the temperature regulating valve 7 is opened to permit air heated in the air heating passage 5 by the hot exhaust passage 4 to be fed to the air cleaner 1 and at the same time to reduce the effective passage area of the outside air induction duct 2 to decrease the amount of cool outside air entering the air cleaner 1.
As a result, the temperature of intake air during cold engine running is raised relative to the outside air so as to effectively prevent back fires resulting from a leaner-than-normal air-fuel ratio of the mixture to be fed to the engine or from an increased mass flow rate of the mixture, which would otherwise be caused by the introduction of intake air at a low temperature during cold running of the engine. Accordingly, heating the intake air serves to maintain a good operating performance of the engine during such cold engine operation.
On the other hand, when the engine is operated at high outside air temperatures, the temperature regulating valve 7 is closed to permit outside air to be fed to the air cleaner 1 only through the outside air induction duct 2.
In this manner, the temperature of intake air is regulated to improve the operating performance of the engine during its cold running.
In the conventional intake air heating system as referred to above, however, the air heating passage 5, acting as a source of heated air to be supplied to the air cleaner 1, is formed by attaching a manihold cover 3 comprising a cover member of the plate or sheet type to the outer periphery of the exhaust passage 4 by means of bolts with a predetermined space or clearance therebetween, and a heated-air outlet 13 for connection with the heated-air induction pipe 6, even if provided in any position of the manifold cover 3, is necessarily placed in close proximity to outside air inlets 14 formed around the peripheral portion of the manifold cover 3 mainly at the side edges of the manifold cover 3, so that cool outside air may be instantaneously led into the air cleaner 1 without being sufficiently heated in the air heating passage 5, thus posing a problem that any satisfactory intake air heating function is not achieved during cold engine running. This is particularly true in a case where the clearance between the exhaust passage 4 and the manifold cover 3 is large due to variations in dimension of these members.